1. Field
The present invention relates generally to isolation of a pressure transducer from a fluid medium, the pressure of which is being measured by the transducer. The invention relates more specifically to an apparatus for isolation of a pressure transducer sensing element from the hostile effects of a fluid environment to which the sensing element is exposed for pressure measurement purposes, without compromising or deleteriously affecting the performance characteristics of the transducer.
2. State of the Art
It is common practice to dispose an isolation element and an isolation fluid between a fluid which is being monitored for pressure and pressure changes and the sensing element of the transducer used to conduct the measurements. Isolation elements in the prior art include diaphragms of many designs, and a wide variety of fluids including water and many different hydrocarbon liquids have been employed as isolation fluids.
It is important that any transducer isolation scheme not only protect the transducer from the fluid environment being measured, but that the isolation components not compromise the accuracy, response time and repeatability of the transducer when in use. Although somewhat self-evident, it is also important that an isolation element itself, and its connection to the transducer or housing in which the transducer is placed, be substantially immune to any hostile characteristics of the fluid environment.
Many transducer applications in industry and commerce would benefit from a rugged, durable, non-performance degrading means of isolating a pressure transducer from the fluid medium being measured. The need for such an isolation means has long been recognized in such diverse areas of application as petroleum exploration, aerospace, purified liquid and gas handling, and petrochemical and other industrial processes. Many such applications impose certain limitations on the design and materials which may be employed for an isolation scheme.
First, applications such as downhole use in oil, gas and geothermal well bores may impose size limitations on the transducer employed as well as on any isolation assembly, including an isolation element. Clearances in drill pipe and tubing, added to wall thicknesses necessary for pressure housings capable of protecting electronic instrumentation to pressures well in excess of 20,000 psi, generally dictate that transducer size be limited to an overall diameter not exceeding one inch (1"). Further, due to the necessarily soft and frictionless nature required of an isolation element so as not to compromise the performance characteristics of the transducer, the size and mass of the isolation element must be kept to an absolute minimum in order to minimize orientation sensitivity due to gravity. Orientation sensitivity is particularly intolerable in highly deviated and horizontal wells. Another limitation related to size of the isolation element is the volume of fluid disposed between the isolation element and the sensing element of the transducer which transmits pressure between the two. The larger the fluid volume and more compressible the isolation fluid, the more stroke or travel is required of the isolation element. This trade-off between volume/compressibility and travel reaches the point where the external pressure ceases to be transmitted completely to the sensing element as the volume of fluid becomes overly large.
Material selection for the isolation element is also somewhat limited by the hostile environment to which the transducer is intended to be exposed. Many environments, for reasons of corrosive effects and conductivity, must be kept from contacting the sensing element itself. Examples of such hostile environments include hydrogen sulfide, carbon dioxide, oxygen, water, and various solvents, some of which readily permeate thin membranes of all known elastomers and also attack many common metals.
Even with the use of corrosion-resistant materials, serviceability of the isolation element is desirable so that it can be cleaned and replaced if necessary, as even corrosion-resistant metals, particularly if of thin cross-section, deteriorate over time when subjected to highly corrosive fluids. Elevated temperatures, such as are experienced in well bores and in many industrial processes, accelerate such deterioration. In addition to corrosion-induced deterioration, most isolation schemes are subject to performance degradation due to particulate contamination. Since, in order to assure optimum transducer performance, the isolation element must be as nearly frictionless and as repeatable as possible, any particulate contamination which interferes with the active or movable part of the isolation element poses the threat of increased friction and interference with travel, each and both of which reduce repeatability.
Finally, a transducer must be easily assembled with its isolation element and isolation fluid, the "dead volume" portion of the assembly between the isolation element and sensing element being completely purgeable of all entrapped gases due to the high compressibility of gases relative to liquids. Furthermore, the dead volume must be repeatable as the transducer is assembled and reassembled over a period of time as isolation elements are replaced. Without such repeatability, which assures the same spring constant and travel range for the isolation element time after time, the transducer would have to be recalibrated throughout its entire range with each isolation element replacement.
Therefore, in summary, it may be said that there has been a long-felt and unmet need for a transducer isolation means of small size and volume, constructed of a rugged, corrosion-resistant material and of a design and construction promoting serviceability and ease of assembly and reassembly with a repeatable end result in terms of performance.
One commonly employed isolation element is a bellows-type diaphragm, various examples of which are disclosed in U.S. Pat. Nos. 1,841,848; 4,077,882; 4,109,531; 4,218,925; 4,461,180; 4,507,972; 4,713,969; U.K. Patent 578,632; and U.K. Patent Application 2106248. However, all such prior art bellows-type isolation schemes are deficient in one or more of the above-enumerated desired characteristics.